Almost universally, a thin elastomeric diaphragm is used to close an opening in a regulator case so that the pressure of air in a breathing chamber is equalized to that of the ambient water. As the diver withdraws air from the breathing chamber, the air pressure in the breathing chamber reduces, causing inward movement of the diaphragm. An actuator is thereby caused to move and to open a demand valve. Air pressure is restored. Under equilibrium conditions, the pressure in the chamber, supplemented by a small biasing force such as by a return spring, equals the pressure of the water, whatever the water pressure may be. By equalizing the pressure of supplied air to that of the surrounding water, the diver's breathing effort is minimized.
Certain patent and other literature stresses the desirability of locating the control diaphragm at the level of the center of pressure of the diver's lungs. The literature also emphasizes the desirability of sensitivity and low friction characteristics of the demand valve itself in order that the biasing force can be minimized, thereby to achieve very close equality between breathing chamber pressure and ambient water pressure.
In order to minimize friction, one prior art device uses a polished metal plate partially embedded in the diaphragm for sliding engagement with an arced or curled end of an actuator lever. The usual procedure is to place the plate in the diaphragm mold so that a circular lip on the inside of the diaphragm surrounds the rim of the plate. The molding procedure requiring the plate to be a mold insert is tedious and expensive.